A lymphokine, called osteoclast activating factor (OAF), has been identified by bioassay, and is postulated to be the cause of hypercalcemia in some hematologic malignancies. A monoclonal antibody to OAF has been produced, and we are using this antibody, in conjunction with immunoenzymatic staining techniques, to determine the role of OAF in the hypercalcemia seen in a significant number of T-cell lymphoma patients. This antibody will also be used to investigate the production of OAF by other normal and neoplastic lymphoid cells.